Catalyst and process for preparing chlorine by gas phase oxidation of hydrogen chloride

ABSTRACT

The present invention relates to a catalyst and to a process for preparing chlorine by catalytic oxidation of hydrogen chloride. The catalyst comprises an active component and a support material, said active component comprising at least uranium or a uranium compound. The catalyst is notable for a high stability and activity at a lower cost compared to the noble metals.

The present invention relates to a catalyst and to a process for preparing chlorine by catalytic oxidation of hydrogen chloride. The catalyst comprises an active component and a support material, the active component comprising at least uranium or a uranium compound. The catalyst is notable for a high stability and activity combined with a lower cost compared to the noble metals.

A reaction of great industrial interest is the process, developed by Deacon in 1868, for catalytic hydrogen chloride oxidation with oxygen:

4HCl+O₂

2Cl₂+2H₂O.

In the past, chloralkali electrolysis forced the Deacon process very much onto the sidelines. Almost the entire production of chlorine was accomplished by electrolysis of aqueous sodium chloride solutions [Ullmann Encyclopedia of industrial chemistry, seventh release, 2006, p 3]. However, the attractiveness of the Deacon process has increased again in recent times, since the global chlorine demand is growing more rapidly than the demand for sodium hydroxide solution. This development is very favourable to the process for preparing chlorine by oxidation of hydrogen chloride, which is decoupled from the production of sodium chloride solution. Furthermore, hydrogen chloride is obtained as a coproduct in large amounts, for example, in phosgenation reactions, for instance in isocyanate preparation.

The oxidation of hydrogen chloride to chlorine is an equilibrium reaction. The equilibrium position shifts away from the desired end product with increasing temperature. It is therefore advantageous to use catalysts with maximum activity which allow the reaction to proceed at low temperature.

The first catalysts for hydrogen chloride oxidation with the catalytically active component ruthenium were described as early as 1965 in DE 1 567 788, in this case proceeding from RuCl₃.

Further Ru-based catalysts with the active component composed of ruthenium oxide or ruthenium mixed oxide have been described in DE-A 197 48 299. In this case, the content of ruthenium oxide is 0.1% by weight to 20% by weight, and the mean particle diameter of ruthenium oxide 1.0 nm to 10.0 nm.

Further Ru catalysts supported on titanium oxide or zirconium oxide are known from DE-A 197 34 412. For the preparation of the ruthenium chloride catalysts described therein, which comprise at least one compound from titanium dioxide and zirconium dioxide, a series of Ru starting compounds have been specified, for example ruthenium-carbonyl complexes, ruthenium salts of inorganic acids, ruthenium-nitrosyl complexes, ruthenium-amine complexes, ruthenium complexes of organic amines or ruthenium-acetylacetonate complexes. In a preferred embodiment, titanium dioxide in the form of rutile was used as the support.

Although the known Ru catalysts already have quite a high activity, a further enhancement of the activity combined with good long-term stability is desirable for an industrial application in hydrogen chloride oxidation. An increase in the reaction temperature would be able to enhance the activity of the Ru catalysts, but they tend to sinter and hence to deactivate at these higher temperatures.

It is known that uranium oxides are suitable as oxidation catalysts for a series of complete and selective oxidations. A typical example of the use of uranium-based catalysts is the oxidation of CO to CO₂, as described, for example, by Campbell et al. in J. Molec. Cat. A: Chem., (2006), 245(1-2), 62-68. Further known oxidations catalysed by uranium-containing mixed oxides are, for example, that of isobutene to acrolein (Corberan et al. Ind. Eng. Chem. Prod. Res. Dev., (1984), 24, 546, and 1985, 24, 62) and that of propylene to acrolein and acrylonitrile (U.S. Pat. No. 3,308,151 and U.S. Pat. No. 3,198,750). Additionally known is the total oxidation of VOCs (volatile organic compounds) over U₃O₈, which has been studied especially by Hutchings et al. (Nature, (1996), 384, p 341). Suitability of uranium compounds for catalytic hydrogen chloride oxidation with oxygen is, however, not disclosed in this connection.

DE 1 078 100 discloses catalysts comprising salts or oxides of silver, uranium or thorium, which are present on inert supports composed of kaolin, silica gel, kieselguhr or pumice. It is not disclosed that the resulting catalysts are calcined, as a result of which a low stability of the catalysts disclosed is to be expected. What is always disclosed is a composition which requires the presence of silver and salts or oxides of rare earths. Accordingly, for the lack of further disclosure, it has to be assumed that the technical teaching is aimed at a cocatalytic effect which only enables a conversion in the interaction of the individual catalytically active constituents.

This is disadvantageous because both the use of silver and of the salts or oxides of the rare earths lead to the catalyst being economically disadvantageous compared to alternatives without these constituents. Especially the use of silver can be considered here to be particularly disadvantageous in view of the continuously rising costs of this noble metal.

It was accordingly an object of the present invention to provide a catalyst which accomplishes the oxidation of hydrogen chloride with high activities combined with good long-term stability and minimum costs. It was a further object of the present invention to provide a process for catalytic gas phase oxidation of hydrogen chloride with oxygen using such a catalyst.

It has now been found that, surprisingly, uranium-based catalysts have a high activity combined with good long-term stability for the oxidation of hydrogen chloride to chlorine. At the same time, uranium-based catalysts offer economic advantages, since they are cheaper than the substances conventionally used in the prior art.

The present invention therefore provides a catalyst for catalytic oxidation of hydrogen chloride, characterized in that it comprises, as catalytically active components, at least uranium or a uranium compound and a support material.

Suitable support materials for the catalyst are, for example, silicon dioxide, aluminium oxide (e.g. in α or γ polymorphs), titanium dioxide (in the form of rutile, anatase, etc.), tin dioxide, zirconium dioxide, cerium dioxide, carbon nanotubes, or mixtures thereof.

Suitable uranium compounds are, for example, uranium oxides, uranium chlorides and uranium oxychlorides. Suitable uranium oxides are, for example—without being restricted thereto—UO₃, UO₂, UO or nonstoichiometric phases resulting from the mixtures, for example U₃O₅, U₂O₅, U₃O₇, U₃O₈, U₄O₉ and/or U₁₃ ^(O) ₃₄.

Preference is given to uranium oxides or mixtures of uranium oxides with a stoichiometric composition of UO_(2.1) to UO_(2.9).

These preferred catalysts comprising uranium oxide or mixtures of uranium oxide as catalysts are particularly advantageous because they surprisingly have an exceptionally high activity and stability for oxidation reactions.

In a preferred embodiment, the precursors used for the uranium oxides may also be the chloride compounds or the oxychloride compounds (UO_(x)Cl_(y)).

The uranium or the uranium compound can be used alone or together with further catalytically active components. Suitable further catalytically active components are those selected from the list comprising ruthenium, osmium, rhodium, iridium, palladium, platinum, copper, silver, gold, rhenium, bismuth, cobalt, iron, antimony, tin, manganese and chromium. Likewise suitable are mixtures thereof or chemical compounds comprising at least one of the abovementioned elements in the list. In a preferred embodiment, ruthenium, gold, bismuth, cerium or Zr and compounds thereof are used. In a very preferred embodiment, ruthenium is used in oxidic form or as a chloride compound or as an oxychloride compound.

Typically, the proportion of the active component is in the range of 0.1 to 90% by weight, preferably in the range of 1 to 60% by weight, more preferably in the range of 1 to 50% by weight, based on the total mass of active component and support material.

The active component can be applied to the support material by various methods. For example, moist and wet impregnation of a support material with suitable starting compounds present in solution or starting compounds in liquid or colloidal form, precipitation and coprecipitation processes, and also ion exchange and gas phase coating (CVD, PVD), can be used. Suitable catalysts can, for example, be obtained by applying uranium or uranium compounds to the support material and then drying, or drying and calcining.

Preference is given to a catalyst in which the active component is applied to the support material in the form of an aqueous solution or suspension and the solvent is then removed.

Particular preference is given to a combination of impregnation and subsequent drying/calcination or a precipitation with reducing substances (preferably hydrogen, hydrides or hydrazine compounds) or alkaline substances (preferably NaOH, KOH or ammonia).

In a further embodiment, the active component can be applied to the support material in a nonoxidic form and be converted to the oxidized form in the course of the reaction.

Particular preference is given to a catalyst which is characterized in that the catalytically active constituent is applied to the support as an aqueous solution or suspension of uranium halides, oxides, hydroxides or oxyhalides, uranyl halides, oxides, hydroxides, oxyhalides, nitrates, acetates or acetylacetonates, in each case alone or in any mixture, and the solvent is then removed.

The catalyst may comprise promoters as a further component. Useful promoters include basic metals (e.g. alkali metals, alkaline earth metals and rare earth metals); preference is given to alkali metals, especially Na and Cs, and alkaline earth metals; particular preference is given to alkaline earth metals, especially Sr, Ba, and the rare earth metal Ce. The promoters may, without being restricted thereto, be applied to the catalyst by impregnation and CVD methods; preference is given to an impregnation, especially preferably after application of the catalytic main component.

The catalyst may comprise, as a further component, compounds for stabilizing the dispersion of the active component. Suitable dispersion stabilizers are, for example, scandium compounds, manganese oxides and lanthanum oxides. The compounds for stabilizing the dispersion are preferably applied together with the active component by impregnation and/or precipitation.

The catalysts can be dried under standard pressure or preferably under reduced pressure under nitrogen, argon or air atmosphere at a temperature of 40 to 200° C. The drying time is preferably 10 min to 6 h.

The catalysts can be used in uncalcined or calcined form. The calcination can be effected in a reducing, oxidizing or inert phase; preference is given to calcination in an air stream or in a nitrogen stream. Calcination is effected typically with exclusion of oxygen in a temperature range of 150 to 100° C., preferably in the range of 200 to 1100° C. In the presence of oxidizing gases, the calcination is effected within a temperature range of 150 to 1500° C., preferably in the range of 200 to 1100° C.

In a preferred further development of this invention, the catalyst comprising uranium or a uranium compound can be subjected to a pretreatment.

The pretreatment is typically a pretreatment under the process conditions of use of the catalyst. Since the catalysts disclosed here are preferably used in the oxidation of HCl with oxygen, a pretreatment with a stoichiometric mixture of oxygen and HCl is preferred. Particular preference is given to a pretreatment with a stoichiometric mixture of HCl and oxygen at least 400° C., preferably at least 500° C. The pretreatment is effected typically at least for 10 h, preferably at least for 50 h, more preferably at least for 100 h.

The pretreatment can be effected at any temperatures for as long as desired. It has been found that a relatively long and relatively hot pretreatment is better than a relatively short and relatively cold pretreatment. Relatively short and relatively cold pretreatments are also conceivable. It is a question of considering to what extent extra expenditure in the pretreatment which is reflected in enhanced activity can be compensated for by this gain in activity. Therefore, the temperature ranges and durations just specified should be understood as sensible recommendations, but not as technical restrictions.

The catalyst obtained as above is notable for a high activity in hydrogen chloride oxidation at low temperature. Without being bound to a theory, it is assumed that uranium oxides form “oxygen defect lattice sites” and can thus actively promote redox cycles. At the same time, uranium oxides have a high stability with respect to hydrogen chloride. This is especially true of the particularly preferred uranium oxides or mixtures of uranium oxides with a stoichiometric composition of UO_(2.1) to UO_(2.9).

The present invention further provides a process for preparing chlorine by catalytic oxidation of hydrogen chloride in the presence of a catalyst which comprises an active component and a support material, characterized in that the active component comprises uranium or a uranium compound.

When the process according to the invention is performed, hydrogen chloride is oxidized with oxygen to chlorine in an exothermic equilibrium reaction in the presence of the catalyst which has already been described above, which affords steam. The reaction temperature is typically 150 to 750° C.; the customary reaction pressure is 1 to 25 bar. Since it is an equilibrium reaction, it is advisable to work at minimum temperatures at which the catalyst still has sufficient activity. Moreover, it is appropriate to use oxygen in superstoichiometric amounts relative to hydrogen chloride. For example, a two- to four-fold oxygen excess is customary. Since there is no risk of any selectivity losses, it may be economically advantageous to work at relatively high pressure and accordingly with a longer residence time compared to standard pressure.

The catalytic hydrogen chloride oxidation can be carried out adiabatically or preferably isothermally or approximately isothermally, batchwise but preferably continuously, as a fluidized bed or fixed bed process, preferably as a fixed bed process, more preferably in tube bundle reactors over heterogeneous catalysts at a reactor temperature of 180 to 750° C., preferably 200 to 650° C., more preferably 220 to 600° C., and a pressure of 1 to 25 bar (1000 to 25 000 hPa), preferably 1.2 to 20 bar, more preferably 1.5 to 17 bar and especially 2.0 to 15 bar.

Typical reaction apparatus in which the catalytic hydrogen chloride oxidation is performed includes fixed bed or fluidized bed reactors. The catalytic hydrogen chloride oxidation can preferably also be carried out in a plurality of stages.

In the adiabatic, the isothermal or the approximately isothermal mode of operation, it is also possible to use a plurality of, i.e. 2 to 10, preferably 2 to 6, more preferably 2 to 5 and especially 2 to 3 reactors connected in series with intermediate cooling. The hydrogen chloride can either be added completely together with the oxygen upstream of the first reactor or divided between the different reactors. This series connection of individual reactors can also be combined in one apparatus.

A further preferred embodiment of an apparatus suitable for the process consists in using a structured catalyst bed in which the catalyst activity rises in flow direction. Such a structuring of the catalyst bed can be accomplished by different impregnation of the catalyst supports with active component or by different dilution of the catalyst with an inert material. The inert materials used may, for example, be rings, cylinders or spheres of titanium dioxide, zirconium dioxide or mixtures thereof, aluminium oxide, steatite, ceramic, glass, graphite or stainless steel. In the case of the preferred use of shaped catalyst bodies, the inert material should preferably have similar external dimensions.

Suitable shaped catalyst bodies include shaped bodies with any shapes; preference is given to tablets, rings, cylinders, stars, wagonwheels or spheres; particular preference is given to spheres, rings, cylinders or star extrudates as the shape.

The catalyst can be shaped after or preferably before the impregnation of the support material.

The conversion of hydrogen chloride in single pass can be limited preferably to 15 to 90%, preferably 40 to 85%, more preferably 50 to 70%. Unconverted hydrogen chloride can, after removal, be recycled partly or fully into the catalytic hydrogen chloride oxidation. The volume ratio of hydrogen chloride to oxygen at the reactor inlet is preferably between 1:1 and 20:1, preferably between 2:1 and 8:1, more preferably between 2:1 and 5:1.

The heat of reaction of the catalytic hydrogen chloride oxidation can advantageously be utilized to raise high-pressure steam. This can be utilized to operate a phosgenation reaction and/or distillation columns, especially isocyanate distillation columns.

The examples which follow illustrate the present invention, but without restricting it thereto.

EXAMPLES Example 1 Uranium Oxide Catalyst on Tin(IV) Support

In a beaker, 2 g of tin (IV) oxide spheres (from Saint-Gobain, BET surface area of 44.6 m²/g) were impregnated with an approx. 10% by weight aqueous solution of uranyl acetate dihydrate (from Riedel-de-Haen) by means of spraying and subsequent weighing, so as to give rise to, by conversion, a loading of 2% by weight of U on the support. After a wait time of 60 min, the catalyst was dried in an air stream at 80° C. for 2 h. The subsequent calcination was effected at 800° C. in an air stream for four hours, which afforded a uranium oxide catalyst supported on tin(IV) oxide.

Example 2 Uranium Oxide Catalyst on TiO₂ Support

In a beaker, 2 g of titanium dioxide (BET of 8.8 m²/g, from Saint-Gobain) were impregnated with an approx. 10% by weight aqueous solution of uranyl acetate dihydrate (from Riedel-de-Haen) analogously to Example 1, so as to give rise to, by conversion, a loading of 2% by weight of U on the support. After a wait time of 60 min, the catalyst was dried in an air stream at 80° C. for 2 h. The subsequent calcination was effected at 800° C. in an air stream for four hours, which afforded a uranium oxide catalyst supported on titanium(IV) oxide.

Example 3 Uranium Oxide Catalyst on Alpha-Al₂O₃ Support

In a beaker, 2 g of alpha-Al₂O₃ (BET of 0.02 m²/g, from Saint-Gobain) were impregnated with an approx. 10% by weight aqueous solution of uranyl acetate dihydrate (from Riedel-de-Haen) analogously to Example 1, so as to give rise to, by conversion, a loading of 2% by weight of U on the support. After a wait time of 60 min, the catalyst was dried in an air stream at 80° C. for 2 h. The subsequent calcination was effected at 800° C. in an air stream for four hours, which afforded a uranium oxide catalyst supported on alpha-aluminium oxide.

Example 4 Uranium Oxide Catalyst on Gamma-Al₂O₃ Support

In a beaker, 2 g of gamma-Al₂O₃ shaped bodies (BET of 260 m²/g, from Saint-Gobain) were impregnated with a 10% by weight aqueous solution of uranyl acetate dihydrate (from Riedel-de-Haen) analogously to Example 1. After an action time of 1 h, the residual water was removed in an air stream at 80° C. for 2 h. The procedure was repeated until 12% by weight of uranium were present on the shaped bodies.

The shaped bodies were subsequently calcined in an air stream at 800° C. for four hours.

Example 5 Uranium Oxide Catalyst on Al₂O₃ Support with Calcination at 800° C., and Analysis

Analogously to Example 4, 40 g of shaped bodies of gamma-Al₂O₃ (BET of 200 m²/g, from Saint-Gobain) were impregnated with uranium and calcined.

Analysis by means of XRD (SIEMENS D 5000 theta/theta reflection diffractometer) showed the presence of gamma-Al₂O₃, and also U₃O₈.

Example 6-8 Use of the Catalysts from Examples 1-3 in HCl Oxidation at 500° C.

0.2 g of the catalysts obtained according to Example 1-3 was ground and introduced into a quartz reaction tube (diameter ˜10 mm) as a mixture with 1 g of quartz sand (100-200 μm).

The quartz reaction tube was heated to 500° C. and then operated at this temperature.

A gas mixture of 80 ml/min of HCl and 80 ml/min of oxygen was passed through the quartz reaction tube. After 30 minutes, the product gas stream was passed into a 16% by weight potassium iodide solution for 10 minutes and the iodine thus formed was back-titrated with a 0.1 N thiosulphate solution in order to determine the amount of chlorine introduced.

This gave the productivities of the catalysts at 500° C. shown in Table 1.

Example 9 Use of the Catalyst from Example 4 in HCl Oxidation at 540° C.

An experiment analogous to those of Examples 6-8 was carried out for the catalyst according to Example 4, except that the quartz reaction tube was now heated to 540° C. and then operated at this temperature.

This gave the productivity of the catalyst at 540° C. shown in Table 2.

Example 10 Use of the Catalyst from Example 4 in HCl Oxidation at 600° C.

An experiment analogous to those of Examples 6-8 was carried out for the catalyst according to Example 4, except that the quartz reaction tube was now heated to 600° C. and then operated at this temperature.

This gave the productivity of the catalyst at 600° C. shown in Table 2.

TABLE 1 Catalyst Productivity at according to 500° C. Example example [kg_(Cl2)/kg_(cat) * h] 6 1 2.28 7 2 0.72 8 3 3.83

TABLE 2 Catalyst Productivity at Productivity at according to 540° C. 600° C. Example example [kg_(Cl2)/kg_(cat) * h] [kg_(Cl2)/kg_(cat) * h] 9.10 4 5.93 9.53 

1. A catalyst for catalytic oxidation of hydrogen chloride, comprising as catalytically active components, at least uranium or a uranium compound and a support material.
 2. The catalyst according to claim 1, wherein the support comprises UO₃, UO₂, UO or nonstoichiometric phases resulting from mixtures of the following species: U₃O₅, U₂O₅, U₃O₇, U₃O₈, U₄O₉, U₁₃O₃₄.
 3. The catalyst according to claim 1, wherein the uranium or the uranium compound is present in the form of uranium oxide or a mixture of uranium oxides with a stoichiometric composition of UO_(2.1) to UO_(2.9).
 4. The catalyst according to claim 1, comprising further substances suitable as catalytically active components, selected from the group consisting of ruthenium, osmium, rhodium, iridium, palladium, platinum, copper, silver, gold, rhenium, bismuth, cobalt, iron, antimony, tin, manganese, chromium, mixtures thereof, and chemical compounds comprising at least one of the abovementioned elements in the list.
 5. The catalyst according to claim 1, wherein the support material is selected from the group consisting of silicon dioxide, aluminium oxide, titanium dioxide, tin dioxide, zirconium dioxide, cerium dioxide, carbon nanotubes, and mixtures thereof.
 6. A process for preparing chlorine comprising the step of catalytically oxidizing hydrogen chloride in the presence of a catalyst, said catalyst comprising an active component and a support material, and wherein the active component comprises uranium or a uranium compound.
 7. The process according to claim 6, wherein the uranium or the uranium compound is present in the form of uranium oxide or a mixture of uranium oxides having a stoichiometric composition of UO_(2.1) to UO_(2.9).
 8. The process according to claim 6, wherein the proportion of the active component is in the range of 0.1 to 90% by weight, based on the total mass of active component and support material.
 9. The process according to claim 6, wherein the reaction temperature is 150 to 750° C.
 10. The process according to claim 6, wherein the reaction pressure is 1 to 25 bar.
 11. The process according to claim 6, wherein it is performed isothermally or adiabatically, continuously and as a fixed bed process.
 12. The process according to claim 6, wherein the process is performed in two or more stages, in 2 to 10 reactors connected in series with intermediate cooling.
 13. The process according to claim 6, wherein the volume ratio of hydrogen chloride to oxygen at the reactor inlet is between 1:1 and 20:1.
 14. The process according to claim 12 wherein the process is performed in two or more stages in 2 to 3 reactors connected in series with intermediate cooling.
 15. The process according to claim 13 wherein the volume ratio of hydrogen chloride to oxygen at the reactor inlet is between 2:1 and 5:1. 